The present invention generally relates to Josephson integrated circuits and more particularly to a Josephson integrated circuit including therein a resistance element.
Intensive efforts are made on the development of ultra-fast integrated circuits that employ the Josephson junctions. Typically, the Josephson junction is formed by an AlO.sub.x tunneling barrier film sandwiched by a pair of niobium superconducting layers. Such a Josephson integrated circuit generally includes resistance elements, and molybdenum or zirconium is commonly used for the resistance material that forms the resistance element. Particularly, zirconium is preferred for the resistance element as it shows the etching rate that is substantially smaller than that of niobium used for the superconducting interconnection pattern. Thereby, the fabrication of the resistance element that includes the step of patterning the metal layer of resistance material by etching, is substantially simplified.
FIGS. 1A-1D show the conventional process for providing the zirconium resistance element.
Referring to the FIG. 1A, a zirconium layer 11 acting as a resistance strip is first deposited on a silicon substrate by a sputtering process and the like, and patterned subsequently to form a zirconium strip having a desired resistance value. In this process of patterning, the substrate 10 is removed from the deposition apparatus and transported to an etching apparatus. During this transportation, the surface of the zirconium layer 11 is inevitably exposed to the air, and there is formed an oxide film 11a of zirconium on the surface of the zirconium layer 11. Generally, the oxide of zirconium shows a semiconductor characteristic in the room temperature but behaves like an insulator at extremely low temperatures such as 4.2 .degree. K. that is the temperature used for operating the Josephson devices. In other words, the surface of the zirconium layer 11 is entirely covered by the insulating zirconium oxide film. The thickness of this zirconium film 11a may be about 2-3 nm while the thickness of the zirconium strip 11 may be about 100 nm, depending on the desired resistance value of the resistance element. After the transportation to the etching apparatus, the zirconium layer 11 is patterned by a reactive ion etching process (RIE) and the like, using a carbon tetrachloride (CCl.sub.4) etching gas, into a zirconium strip as shown in FIG. 1A. It should be noted that the RIE process using CCl acts on zirconium and niobium with substantially the same etching rate.
On this zirconium strip 11, a niobium superconducting interconnection is deposited. As this oxide film 11a prevents the electrical connection to the zirconium strip 11, the structure of FIG. 1A is first subjected to a sputter-etching process, wherein the zirconium oxide layer 11a is removed by an bombardment of argon ions as shown in FIG. 1B. This sputter-etching process is continued until substantially entire oxide layer 11a is eliminated.
After the removal of the oxide layer 11a, a niobium layer 12 is deposited to bury the zirconium strip 11 underneath as shown in FIG. 1C, and the niobium layer 12 is patterned into a first conductor segment 12a and a second conductor segment 12b that are separated with each other as shown in FIG. 1D, with the zirconium strip 11 intervening between the conductor segment 12a and the conductor segment 12b. This patterning is achieved by an RIE process using carbon tetrafluoride (CF.sub.4) as the etching gas. Thereby, both the first and second conductor segments 12a and 12b are connected electrically to the zirconium strip 11. As the RIE process acts selectively on niobium, the zirconium strip 11 remains substantially intact even when the niobium layer 12 is patterned.
In the foregoing process, it will be immediately understood that the process has a problem in the step of FIG. 1B for removing the oxide film 11a. As the bombardment of argon ions is indiscriminate whether the subject is the oxide film 11a or the zirconium strip 11, there is a substantial risk that the zirconium strip 11 itself is subjected to the sputter-etching process after the oxide film 11a is removed. When this occurs, the resistance value of the resistance element is inevitably deviated from the designed resistance value. At the moment, it is extremely difficult to stop the sputter-etching process exactly at the moment when the top surface of the zirconium strip 11 is exposed. Further, the foregoing conventional structure of FIG. 1D suffers from a problem of time-dependent variation in the resistance value of the resistance element as will be described later with reference to the effect of the present invention.
When molybdenum is used for the resistance strip 11 in place of zirconium, on the other hand, there arises a problem in the step of FIG. 1D for patterning the niobium superconductor layer, because the molybdenum has an etching rate that is substantially identical with the etching rate of niobium. Thus, the patterning process of FIG. 1D would entirely remove away the molybdenum resistance strip 11. In order to avoid this, one has to protect a part of the surface of the strip 11 corresponding to the part exposed in the structure of FIG. 1D by a material such as silicon oxide that is immune to the etching process. However, provision of such a protective region requires a complex deposition and patterning process between the step of FIG. 1B and the step of FIG. 1C and is not desirable.